1. Field of the Invention
The invention relates to multimode optical fibers. More particularly, the invention relates to multimode optical fibers having improved bandwidth and differential mode delay characteristics.
2. Description of the Related Art
The bandwidth of optical fibers, including multimode optical fibers, relates generally to the amount of information the optical fiber transmits per given unit of time. In the frequency domain, bandwidth is defined as the frequency at which the response of the optical fiber (under a particular set of launch condition parameters) is 3 decibels (dB) down from the response of the optical fiber when the modulating frequency is zero or approximately zero. In the frequency domain, bandwidth is measured directly in units of megahertz (MHz), multiplied by the fiber""s length in kilometers (km) and expressed in units of MHz-km.
A major factor affecting the bandwidth of a multimode fiber is the particular shape of the refractive index distribution of the optical fiber""s core with respect to a theoretically perfect index profile. In multimode optical fibers, lower order modes are transmitted closer to the center of the fiber and higher order modes typically travel along longer paths that oscillate back and forth between the center of the fiber core and the core/cladding boundary. Because the lower order modes have a shorter path of travel than higher order modes and thus travel through a multimode fiber sooner, the refractive index of a multimode optical fiber typically is configured to decrease gradually when moving radially outward from the core center to the core/cladding boundary. The gradually decreasing refractive index slows down the travel speed of the lower order modes relative to the higher order modes. The decrease in refractive index should coincide with the gradual increase in travel time of the higher order modes so that, desirably, all the modes arrive at the far end of the multimode optical fiber at the same time, in which case mode dispersion is minimized.
The refractive index of such a fiber, known as graded-index fiber, is given as
n(r)=n(o)[1xe2x88x922xcex94(r/a)xcex1]1/2
where r is radial distance from the fiber axis, xcex1 is the core radius, n(o) is the index maximum, xcex94 is the relative index difference between the core and the cladding, and xcex1 is a power law exponent that characterizes the profile shape. An optical fiber having a refractive index profile generally along the shape of a graded-index fiber typically is said to have an alpha profile or xcex1-profile.
Because of the relationship between an optical fiber""s bandwidth and change in refractive index profile, the ability to predict the bandwidth of a multimode optical fiber based on its index profile has long been a desire. However, relatively small changes in refractive index profiles often generate relatively large effects on multimode optical fiber bandwidth. Therefore, the bandwidth and other characteristics of multimode optical fibers often are characterized instead in terms of their differential mode delay. Differential mode delay refers to the differential or relative time required for various modes within a multimode optical fiber to travel a certain distance.
Other factors that affect bandwidth of a multimode optical fiber include the inherent change in refractive index as a function of wavelength, which causes different wavelengths to travel at different speeds through the optical fiber. Thus, the index profile for improving the bandwidth of the optical fiber at one wavelength differs from the index profile needed to improve the bandwidth of the optical fiber at another wavelength.
Because of these factors and other effects, conventional optical fibers often are configured for optimal bandwidth performance at various operating windows, e.g., the 850 nanometer (nm) window, which coincides with the operating frequency of many early conventional light emitting diode (LED) optical sources and detectors, or the 1300 nm window, which coincides with the operating frequency of many conventional laser sources. Also, it is possible to configure optical fibers for optimal bandwidth performance at other conventional operating windows. It should be remembered that, because of the factors discussed hereinabove, the transmission characteristics of an optical fiber optimized for operation at the 850 nm window are much different and less optimal when the optical fiber is operating in, e.g., the 1300 nm window.
Many conventional applications have evolved around an optical fiber standard performance of 200 MHz-km bandwidth at the 850 nm operating window and 500 MHz-km bandwidth at the 1300 nm operating bandwidth, with the bandwidth being measured using overfill-launch conditions. An overfilled launch attempts to excite all modes, and typically is performed using a LED or other suitable source. The overfilled launch is compared with, e.g., a restricted launch or laser-launch, which attempts to excite one or more specific modes using, e.g., a laser source.
Recent developments have focused on systems employing optical fiber with higher bandwidth capabilities that still perform well under older performance conditions. For example, it is desirable to have available optical fibers configured for optimal laser-launch performance at the 850 nm operating window that also have improved overfill-launch bandwidth at 1300 nm operating window. Similarly, it is desirable to have available optical fibers configured for optimal laser-launch performance at 1300 nm that also have improved overfill-launch bandwidth at the 850 nm operating window.
The invention is embodied in an optical communications system including a multimode optical fiber having improved overfill-launch bandwidth performance without disturbing existing laser-launch bandwidth performance. Embodiments of the invention provide a multimode optical fiber having a characteristic differential mode delay with a first portion associated with lower order modes that behaves conventionally and a second portion associated with higher order modes that deviates from conventional behavior in a way that improves overfill-launch bandwidth performance at one operating window without adversely impacting the laser-launch bandwidth performance at the same and other operating windows. In one embodiment, a multimode optical fiber conventionally optimized for operation at 850 nm is configured or otherwise modified in such a way that, when operating at 1300 nm, the differential mode delay initially increases in a conventional manner and then flattens out to approximately zero (0) for the higher order modes. Also, at 850 nm, the differential mode delay of the multimode optical fiber initially remains flat at approximately zero, in a conventional manner, and then decreases at the higher order modes. However, such decrease does not adversely affect bandwidth performance of laser-launches into predominantly low-order modes because the lower order modes rather than the higher order modes are more important to bandwidth performance.
Alternatively, according to embodiments of the invention, a multimode optical fiber conventionally optimized for operation at 1300 nm is configured to have a differential mode delay that, when operating at 850 nm, initially decreases in a conventional manner at the lower order modes and then flattens out to approximately zero at the higher order modes. When operating at approximately 1300 nm, the differential mode delay begins flat at approximately zero with the lower order modes and then increases slightly at the higher order modes.
Embodiments of the invention include an inventive optical fiber preform and a method for making the optical fiber preform. The inventive preform includes a core region configured to have a refractive index with the characteristic differential mode delay just described, and a cladding region surrounding the core region with a refractive index lower than the refractive index of the core region. The method for making the preform includes providing a core region having a refractive index that will produce an optical fiber having the desired characteristic differential mode delay, and forming a cladding region around the core region. Alternatively, the method for making the preform includes the step of modifying the refractive index of a conventional core region in such a way that the optical fiber formed from the preform has characteristic differential mode delay plots that behave in the desired manner described hereinabove.